Multilevel optical recording medium with calibration signals

ABSTRACT

It is an object of the present invention to provide a multilevel optical recording medium in which influence of variation in characteristics of a recording layer in a plane of the recording layer on reproduced data can be corrected 
     The multilevel optical recording medium according to the present invention includes at least a recording layer  12  and is constituted so that data can be recorded therein in a multilevel manner by controlling a state of the recording layer  12  among multiple stages and the multilevel optical recording medium according to the present invention is characterized in that a plurality of calibration signals are stored in the recording layer  12.  According to the thus constituted multilevel optical recording medium, since an optical data recording and reproducing apparatus can detect and correct for the variation in characteristics of the recording layer  12  in the plane thereof, even in the case where variation in characteristics of the recording layer  12  in the plane thereof is present, data recorded in the multilevel optical recording medium in a multilevel manner can be read in a desired manner.

FIELD OF THE INVENTION

The present invention relates to a multilevel recording medium.

This application is a 371 of PCT/JP02/12732, filed Dec. 4, 2002.

DESCRIPTION OF THE PRIOR ART

Optical recording media typified by the CD and the DVD have been widelyused as recording media for recording digital data, and a widely useddata recording format is a format wherein the lengths of pits along thetrack are modulated depending on the data to be recorded.

In this recording format, when data are to be reproduced, a laser beamset to a reproducing power is projected onto the optical recordingmedium along the tracks thereof and light reflected from the opticalrecording medium is detected, thereby judging whether or not a pit ispresent on the reflection surface. On the other hand, when data are tobe recorded, a laser beam set to a recording power is projected onto theoptical recording medium along the tracks thereof, whereby a pit of apredetermined length is formed.

In recent years, it has become strongly desirable to achieve furtherincreases in the density of data and in order to achieve this, aso-called “multilevel recording format” has been proposed. Unlike theabove mentioned recording format, in the multilevel recording format,one of a plurality of recording marks each with different meanings isassigned to a virtual recording cell. When data are to be reproduced, alaser beam set to a reproducing power is projected onto the opticalrecording medium along the tracks thereof and light reflected from theoptical recording medium is detected, thereby judging the kind ofrecording mark assigned to each of the virtual recording cells. On theother hand, when data are to be recorded, a laser beam set to arecording power is projected onto the optical recording medium along thetracks thereof, whereby a recording mark to be assigned to each of thevirtual recording cells is recorded in the virtual recording cell.

The virtual recording cells to which different recording marks areassigned have different light transmittances with respect to a laserbeam. Specifically, when data are to be recorded, the amount of a laserbeam set to a recording power projected onto each of the virtualrecording cells is controlled in a multilevel manner, whereby the lighttransmittance of each of the virtual recording cells is determined in amultilevel manner.

Here, the “light transmittance” means the ratio of the amount of a laserbeam passing through a virtual recording cell to the amount of the laserbeam projected onto the virtual recording cell when data are to bereproduced. Therefore, when data are to be reproduced, a laser beamprojected onto a virtual recording cell passes through the virtualrecording cell, is reflected by a reflective layer, again passes throughthe virtual recording cell and is emitted from the multilevel recordingmedium to the outside and the intensity of the thus emitted laser beamis detected, thereby judging the kind of a recording mark assigned tothe virtual recording cell.

As apparent from the above, in order to record data in the multilevelrecording medium with higher density, it is effective to control thereflection coefficients of the virtual recording cells among morestages. For example, if the reflection coefficients of the virtualrecording cells are controlled among four stages, information recordedin each of the virtual recording cells is expressed by two bits, whileif the reflection coefficients of the virtual recording cells arecontrolled among eight stages, information recorded in each of thevirtual recording cells is expressed by three bits. In order to enablethe reflection coefficients of the virtual recording cells to becontrolled among more stages, it is necessary for the difference betweenthe reflection coefficient of a virtual recording cell to which arecording mark having the highest light transmittance is assigned andthe reflection coefficient of a virtual recording cell to which arecording mark having the lowest light transmittance is assigned,namely, the dynamic range, to be considerably large. Therefore, arecording layer which can ensure a considerably wide dynamic range isselected as the recording layer of the multilevel recording medium. Forthis purpose, an organic dye recording layer or a phase change typerecording layer is selected as the recording layer of the multilevelrecording medium and is formed using a spin coating process or asputtering process.

The spin coating process is used as a process for applying a coatingmaterial in a liquid form not only in the field of manufacturing opticalrecording media but also in the field of manufacturing semiconductordevices and the like. However, when a film is formed using the spincoating process, although the thickness of the film can be madesubstantially uniform at regions spaced a distance outward from thepoint at which the coating solution is dropped (the dropping point), ittends in the vicinity of the dropping point to thin rapidly withincreasing proximity to the dropping point. Therefore, thecharacteristics of the recording layer inevitably vary depending uponthe planar positions in the recording layer.

On the other hand, the sputtering process is generally used for forminginorganic material films, including the recording layer of an opticalrecording medium, but the distribution of film thickness variesdepending upon the planar positions in the radial direction orcircumferential direction of a disk-like optical recording mediumbecause of the structure of the sputtering device, the influence ofsputtering target consumption (erosion) as the sputtering processprogresses, and the like.

It is possible to reduce such variation in thickness of the recordinglayer in the plane thereof to some extent by improving the manufacturingequipment and/or the manufacturing process. However, even thoughvariation in thickness of the recording layer may not be a seriousproblem in the recording format used for the CD and the DVD wherein thelengths of pits along the track are modulated depending on the data tobe recorded, even a small variation in thickness of the recording layermakes it difficult to reproduce data recorded in the multilevelrecording medium because it is necessary to control the reflectioncoefficients of the virtual recording cells in a multilevel manner. Inparticular, in the case where an organic dye recording layer is used asthe recording layer, since the recording layer is formed using the spincoating process and the variation in thickness of the recording layer inthe plane thereof is large, it is necessary for the data reproducingapparatus to detect and correct the influence of the variation inthickness of the recording layer in the plane thereof on reproduced datain order to reproduce data recorded therein in a desired manner.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amultilevel recording medium in which influence of variation incharacteristics of a recording layer in a plane of the recording layeron reproduced data can be corrected.

The above object of the present invention can be accomplished by amultilevel recording medium which comprises at least a recording layerand in which data can be recorded in a multilevel manner by controllinga state of the recording layer among multiple stages, a plurality ofcalibration signals being stored in the recording layer.

According to the present invention, since a plurality of calibrationsignals are stored in the recording layer, a data reproducing apparatuscan detect the variation in thickness of the recording layer in theplane thereof and correct it. Therefore, even in the case wherevariation in thickness of the recording layer in the plane thereof ispresent, data recorded in the recording layer in a multilevel manner canbe reproduced in a desired manner.

In a further preferred aspect of the present invention, the calibrationsignals include at least a first virtual recording cell sequence inwhich a plurality of virtual recording cells each recorded with firstinformation are formed and a second virtual recording cell sequence inwhich a plurality of virtual recording cells each recorded with secondinformation different from the first information are formed.

In a further preferred aspect of the present invention, the firstinformation is information corresponding to a virtual recording cellhaving the highest reflection coefficient and the second information isinformation corresponding to a virtual recording cell having the lowestreflection coefficient.

In a preferred aspect of the present invention, the recording layer isformed of a dye.

In a further preferred aspect of the present invention, the recordinglayer is formed using a spin coating process.

In a further preferred aspect of the present invention, density ofcalibration signals stored at an inner circumferential portion is higherthan that of calibration signals stored at an outer circumferentialportion.

According to this preferred aspect of the present invention, it ispossible to further improve the accuracy of reading data and to ensure alarger region usable by a user.

In another preferred aspect of the present invention, the recordinglayer is formed of a phase change material.

In another preferred aspect of the present invention, the recordinglayer is formed using a sputtering process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partially cutaway perspective view showing theconfiguration of a multilevel optical recording medium 1.

FIG. 2 is a diagram schematically showing recording marks Ma to Mgrecorded in an optical recording medium 1.

FIG. 3 is a graph showing relative light reflection coefficientcharacteristics of optical recording media 1 fabricated using variousorganic dyes.

FIG. 4 is a graph schematically showing the relationship between thethickness of a recording layer 12 and distance in the diameterdirection.

FIG. 5 is a diagram schematically showing a method for defining a lightreflection coefficient using calibration signals.

FIG. 6 is a diagram schematically showing a preferred example of avirtual recording cell train 20 constituting calibration signals.

FIG. 7 is a block diagram schematically showing the structure of anoptical data recording and reproducing apparatus 40.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explained indetail with reference to the drawings.

FIG. 1 is a schematic partially cutaway perspective view showingconfiguration of a multilevel optical recording medium 1 (hereinaftersometimes referred to as “optical recording medium 1”).

In this embodiment, the optical recording medium 1 is constituted as aCD-R type optical recording medium (a write-once type optical recordingmedium). As shown in FIG. 1, the optical recording medium 1 includes asubstrate 11, a recording layer 12, a reflective layer 13 and aprotective layer 14. The substrate 11 is disc-like and formed of atransparent resin. Grooves 11 a and lands 11 b for guiding a laser beamare spirally formed on one of the surfaces of the substrate 11 (theupper surface thereof in FIG. 1) so as to extend from a portion in thevicinity of the center of the substrate 11 toward the outercircumference thereof. The recording layer 12 is formed of an organicdye such as cyanine dye, merocyanine dye, methine system dye,derivatives thereof, benzenethiol metal complex, phthalocyanine dye,naphthalocyanine dye, azo dye or the like and the grooves 11 a and thelands 11 b are covered with the organic dye applied thereonto. When alaser beam set to a recording power by an optical data recording andreproducing apparatus is projected onto the recording layer 12, therecording layer 12 is decomposed and degraded and light transmittancethereof is changed in accordance with the amount of the laser beamprojected thereonto. Since the recording layer 12 is formed of theorganic dye, it is formed using a spin coating process. The reflectivelayer 13 is a thin layer for reflecting the laser beam passing throughthe substrate 11 and the recording layer 12 when data recorded in theoptical recording medium 1 are to be reproduced. It is formed bysputtering a material containing metal such as Au, Ag or the like on therecording layer 12, for example. The protective layer 14 serves toprotect the reflective layer 13 and the recording layer 12 and is formedso as to cover the outer surface of the reflective layer 13.

However, the optical recording medium according to the present inventionis not limited to the above mentioned CD-R type optical recording medium(write-once type optical recording medium) and may be a CD-W typeoptical recording medium (data rewritable type optical recording medium)in which a phase change material is used for a recording layer. In thiscase, it is necessary to form a lower protective layer (dielectriclayer) between the recording layer and the reflective layer and an upperprotective layer (dielectric layer) between the recording layer and thesubstrate. The recording layer, the lower protective layer and the upperprotective layer can be formed using a sputtering process. A GeSbTematerial or an AgInSb material may be used as a phase change materialfor forming the recording layer.

Next, the principle of recording data in the optical recording medium 1will be explained below with reference to a drawing.

As shown in FIG. 1, in the optical recording medium 1, virtual recordingcells S, S, . . . , each of which constitutes a recording unit, aredefined by virtually dividing the grooves 11 a along the rotationdirection (circumferential direction) of the optical recording medium 1.

FIG. 2 is a diagram schematically showing recording marks Ma to Mgrecorded in the optical recording medium 1. As shown in FIG. 2, thelength of each of the virtual recording cells S along the grooves 11 ais determined to be shorter than the diameter D of a condensed beam(diameter of the beam waist). Each of the virtual recording cells S isonly an assumed virtual cell on the side of the optical data recordingand reproducing apparatus and no boundaries for defining the individualvirtual recording cells are present in the optical recording medium 1.

As shown in FIG. 2, recording marks Ma to Mg (hereinafter sometimesreferred to as “recording marks M” when the recording marks are notdistinguished from each other) in which the degrees of decomposition anddegradation of the recording layer 12 (mainly, the organic dye) aredifferent from each other are formed in the virtual recording cells S bycontrolling the time period during which a laser beam is emitted from apick up of the optical data recording and reproducing apparatus whendata are to be recorded in the recording layer 12, namely, the amount ofthe laser beam projected onto the recording layer 12, in a multilevelmanner in accordance with values of data to be recorded. Here, in FIG.2, the degrees of decomposition and degradation of the recording layer12 are schematically indicated by the lengths of the recording marks.When data are to be recorded using a laser beam, since the laser beam isprojected onto the optical recording medium 1 while it is being rotated,each of the recording marks M has an elliptic shape whose lengthcorresponds to the time period during which a laser beam is projectedonto the optical recording medium 1.

Therefore, when data are to be recorded in the optical recording medium1 in a multilevel manner, the degrees of decomposition and degradation(change in light transmittances) of the record marks Ma to Mg aredetermined so that the light reflection coefficients of the record marksMa to Mg when a laser beam is projected onto the virtual recording cellsS for reproducing data vary among seven stages (eight stages if anunrecorded region is counted), for example. In this case, the lightreflection coefficient of the record mark increases as the degree ofdecomposition and degradation of the recording layer 12 is lower. As aresult, the virtual recording cell S to which no recording mark M isassigned has the highest light reflection characteristic and the virtualrecording cell S to which the smallest recording mark Ma is assigned hasthe highest light reflection characteristic among the virtual recordingcells S to which the recording marks M are assigned. Further, the lightreflection coefficients of the virtual recording cells S to which therecording marks Mb to Mf decreases from the virtual recording cell S towhich the recording mark Mb is assigned toward the virtual recordingcell S to which the recording mark Mf is assigned and the virtualrecording cell S to which the largest recording mark Mg is assigned hasthe lowest light reflection characteristic. Therefore, it is possible toform the recording marks Ma to Mg whose light reflection coefficientsvary among seven stages by controlling the amount of a laser beamprojected onto the recording layer 12 to appropriately determinedecomposed and degraded areas of the recording layer 12, namely thelight transmittances of the recording layer 12.

Next, the characteristics of the organic dye used for the recordinglayer 12 of the optical recording medium 1 will be described below withreference to the drawings.

The organic dye used for the recording layer 12 generally has suchcharacteristics that the degree of decomposition and degradation thereofincreases as the projection time of a laser beam (the amount of thelaser beam projected thereonto) increases. On the other hand, the lightreflection coefficient of the organic dye does not linearly change withrespect to the projection time of the laser beam (the amount of thelaser beam projected thereonto). Further, the decomposition anddegradation of the organic dye caused by irradiation with the laser beamgradually progresses for a predetermined time period after theprojection of the laser beam is started and, then, after thepredetermined time period has passed, steeply and linearly progresses,whereafter it again progresses gradually until eventually substantiallyleveling off after a predetermined time period has passed.

Further, the light transmittance of the organic dye which is neitherdecomposed nor degraded, the light transmittance of the organic dyewhich has been most greatly decomposed and degraded (the lighttransmittance of the organic dye which has decomposed and degraded tosuch an extent that substantially no further decomposition anddegradation occur even if the laser beam is further projected thereonto)and the change in the light transmittances of the organic dye inaccordance with the degree of decomposition and degradation thereofdepend upon the kind of the organic dye used for the recording layer 12.Therefore, if five kinds of optical recording media 1 are fabricated byforming recording layers 12 using different organic dyes, for example,the absolute light reflection coefficients of the recording layers 12 ofthe optical recording media 1 will be different from each other. Here,the “absolute light reflection coefficient” means the light reflectioncoefficient of an unrecorded region (unrecorded virtual recording cell)of an optical recording medium 1 in the case where the value of thelight reflection coefficient of a disc fabricated by forming a thin filmof Au or the like on a smooth surface thereof using a sputteringprocess, for example, is assumed to be 100%.

FIG. 3 is a graph showing relative light reflection coefficientcharacteristics of optical recording media 1 fabricated using variousorganic dyes.

As indicated by characteristic curves C1 to C5 in FIG. 3, the relativelight reflection coefficient characteristics of recording layers 12 ofthe optical recording media 1 are different from each other. As shown inFIG. 3, the gradients of the characteristic curves C1 to C5 areinfluenced by the degrees of decomposition and degradation of theorganic dyes. Here, the “relative light reflection coefficient” meansthe light reflection coefficient of a recorded region, namely, arecorded virtual recording cell S of an optical recording medium 1, inthe case where the value of the absolute light reflection coefficient ofan unrecorded region, namely, an unrecorded virtual recording cell S ofthe optical recording medium 1, is assumed to be 100%.

Not only do, the absolute light reflection coefficient and the relativelight reflection coefficient depend upon the kind of the organic dye asshown in FIG. 3, they further depend upon the thicknesses of therecording layers even in the case where the same organic dye is used.

FIG. 4 is a graph schematically showing the relationship between thethickness of the recording layer 12 and distance in the diameterdirection.

As shown in FIG. 4, the relationship between the thickness of therecording layer 12 and distance in the diameter direction can beapproximately expressed by a logarithm function and the thickness of therecording layer 12 decreases sharply toward the inner portion of theoptical recording medium 1. Such tendency inevitably occurs when therecording layer 12 is formed using a spin coating process. Therefore,even in the case where the same data are recorded in the same opticalrecording medium 1, the light reflection coefficient depends uponposition in the diameter direction of the optical recording medium 1 andthe light reflection coefficient greatly varies at an innercircumferential region of the optical recording medium 1. This causesvariation in the light reflection coefficients of the recording layer 12in the plane thereof.

Therefore, when data are to be reproduced from the optical recordingmedium 1, it is necessary for the optical data recording and reproducingapparatus to detect and correct for the variation in the lightreflection coefficient of the recording layer 12 in the plane thereofand it is therefore necessary to store a plurality of reference signalsin the optical recording medium 1. The reference signals are stored inthe optical recording medium 1 in the same way that user data arerecorded therein by using the virtual recording cells S. Specifically,the reference signals are constituted by virtual recording cells S inwhich predetermined recording marks are formed and/or unrecorded virtualrecording cells S. The reference signals are used for detecting thevariation in the light reflection coefficient of the recording layer 12in the plane thereof with change in the recording position. For example,in the case where a reference signal is constituted by a virtualrecording cell S to which the largest recording mark Mg is assigned, theoptical data recording and reproducing apparatus again defines thereflection coefficient of the reference signal as the reflectioncoefficient of the virtual recording cell S to which the recording markMg at the recording position is assigned and reads data based on thethus defined reflection coefficient of the virtual recording cell S. Inthis specification, these reference signals are referred to as“calibration signals”.

Next, a method for defining the light reflection coefficient using thecalibration signals will be concretely described below.

FIG. 5 is a diagram schematically showing a method for defining thelight reflection coefficient using the calibration signals.

In FIG. 5, the calibration signals are constituted by the virtualrecording cell S7 to which the recording mark Mg is assigned and anunrecorded virtual recording cell S0 and the calibration signals arestored in recording positions A, B and C. In this case, the lightreflection coefficient of the virtual recording cell S7 to which therecording mark Mg is assigned and the light reflection coefficient ofthe unrecorded virtual recording cell S0 are defined by the calibrationsignals read from the recording position A and the light reflectioncoefficients of the virtual recording cells S1 to S6 to which recordingmarks Ma to Mf are assigned are defined by dividing the differencebetween light reflection coefficient of the virtual recording cell S7 towhich the recording mark Mg is assigned and the light reflectioncoefficient of the unrecorded virtual recording cell S0 into sevenevenly spaced values. As a result, at a region between the recordingposition A and the recording position B, the content of data assigned toeach of the virtual recording cells S is judged by referring to theabove defined light reflection coefficients. For example, in the casewhere the light reflection coefficient obtained from a certain virtualrecording cell S is defined by the calibration signals as a lightreflection coefficient of a virtual recording cell S to which therecording mark Md is assigned, the optical data recording andreproducing apparatus judges that the recording mark Md is assigned tothe virtual recording cell S.

Such calibration signals are also stored at the recording position B andat a region between the recording position B and the recording positionC, the content of data assigned to each of the virtual recording cells Sis judged based on the definition made by the calibration signals andstored at the recording position B.

Thus, the correction is made using the calibration signals, therebycanceling the influence of the variation in the light reflectioncoefficients of the recording layer 12 in the plane thereof caused byforming the recording layer 12 of the organic dye using a spin coatingprocess.

The calibration signals are not particularly limited regarding concretecontent but, as illustrated referring to in FIG. 5, it is preferable toconstitute the calibration signals by a virtual recording cell S whoselight reflection coefficient is lowest and a virtual recording cell Swhose light reflection coefficient is highest. In this case, since thelength of each of the virtual recording cells S along the grooves 11 ais determined to be shorter than the diameter D of the condensed beam(diameter of the beam waist), the light reflection coefficient of eachof the virtual recording cells S is influenced by neighboring virtualrecording cells S. Therefore, in order to suppress the influence of theneighboring virtual recording cells S, it is preferable to constitutethe calibration signals by a plurality of virtual recording cells Shaving the lowest light reflection coefficient and a plurality ofvirtual recording cells S having the highest light reflectioncoefficient.

FIG. 6 is a diagram schematically showing a preferred example of avirtual recording cell train 20 constituting calibration signals.

As shown in FIG. 6, the virtual recording cell sequence 20 constitutingcalibration signals consists of a virtual recording cell train S00consisting of ten successive ten unrecorded virtual recording cells anda virtual recording cell train S77 consisting of ten successive tenvirtual recording cells to which recording marks Mg are assigned. Whenlight reflection coefficients are to be defined using such virtualrecording cell train 20, a laser beam set to a reproducing power isprojected onto the virtual recording cell train 20 and the lightreflection coefficient of the cells included in the virtual recordingcell train S00 and the light reflection coefficient of the cellsincluded in the virtual recording cell train S77 are measured. Then, thelight reflection coefficient of an unrecorded virtual recording cell Sis defined based on the light reflection coefficient of the cellsincluded in the virtual recording cell train S00 and the lightreflection coefficient of a virtual recording cell S to which therecording mark Mg is assigned is defined based on the light reflectioncoefficient of the cells included in the virtual recording cell trainS77. Further, the light reflection coefficients of virtual recordingcells S to which recording marks Ma to Mf are defined by dividing thedifference between light reflection coefficient of the virtual recordingcell S to which the recording mark Mg is assigned and the lightreflection coefficient of the unrecorded virtual recording cell S intoseven equally spaced values.

In this case, when the light reflection coefficient of the unrecordedvirtual recording cell S is to be defined, absolute light reflectioncoefficients obtained from virtual recording cells S other than thefirst virtual recording cell S00-start and the last virtual recordingcell S00-end among the virtual recording cells S included in the virtualrecording cell train S00 are used and when the light reflectioncoefficient of the virtual recording cell S to which the recording markMg is assigned is to be defined, absolute light reflection coefficientsobtained from virtual recording cells S other than the first virtualrecording cell S77-start and the last virtual recording cell S77-endamong the virtual recording cells S included in the virtual recordingcell train S77 are used. As a result, variation in light reflectioncoefficients caused by the influence of the neighboring virtualrecording cells S can be eliminated.

Here, it is sufficient to store the calibration signals for eachpredetermined interval, for example, for each 10 KB to 100 KB of userdata or each 10,000 to 100,000 virtual recording cells S. However,storing the calibration signals at high density has the effect ofimproving data reading accuracy at the expense of considerably reducingthe area usable by the user, while storing the calibration signals atlow density has the effect of minimizing the reduction of the areausable by the user at the expense of lowering the data reading accuracy.Therefore, it is preferable to determine the storage density of thecalibration signals in accordance with the required data readingaccuracy and required data storage capacity.

In particular, it is preferable to vary the density of the calibrationsignals stored in the optical recording medium 1 in accordance with themagnitude of the variation in the light reflection coefficient of therecording layer 12 in the plane thereof. Concretely, as mentionedreferring to FIG. 4, since the variation in thickness of the recordinglayer 12 becomes large toward the inner circumferential portion thereof,it is preferable to determine the storage density of the calibrationsignals larger at regions closer to the inner circumferential portion ofthe recording layer 12 and smaller at regions closer to the outercircumferential portion thereof. If the calibration signals are storedin this manner, accuracy of reading data can be improved and the areausable by the user can be increased in comparison with the case wherethe calibration signals are stored at uniform intervals. In this case,the storage density of the calibration signals may be continuouslyincreased with increasing proximity to the inner circumferential portionof the recording layer 12 or may be stepwise increased with increasingproximity to the inner circumferential portion of the recording layer12.

Next, an optical data recording and reproducing apparatus 40 forrecording data in the optical recording medium 1 and reproducing datafrom the optical recording medium 1 will be described below.

FIG. 7 is a block diagram schematically showing the structure of theoptical data recording and reproducing apparatus 40.

As shown in FIG. 7, the optical data recording and reproducing apparatus40 is constituted as a CD-R recorder and includes a spindle servo 41, aspindle motor 42, a pick up 43, a focus tracking servo 44, a feed servo45 and a control device 46. The spindle motor 42 is driven andcontrolled by the spindle servo 41 so as to rotate the optical recordingmedium 1 at a constant linear velocity. The pick up 43 projects a laserbeam which is emitted from a laser beam source (not shown) driven by alaser driver (not shown) under the control of the control device 46 andwhose power is set to a recording power or a reproducing power onto theoptical recording medium 1. As a result, recording marks M are recordedin virtual recording cells S and electric signals whose intensitiescorrespond to the levels of the laser beam reflected from the virtualrecording cells S are output. In this case, when data are to berecorded, the laser driver of the pick up 43 is controlled by thecontrol device 46 so as to adjust the amount of the laser beam projectedonto a virtual recording cell S, such as by controlling the number ofpulses of the laser beam and/or the power of the laser beam or theheight of pulses of the laser beam in accordance with the content ofdata to be recorded. Instead of adjusting the amount of the laser beamby the laser driver, the amount of the laser beam can be adjusted bydisposing a light modulator in the optical path of the laser beam anddriving and controlling the light modulator using the control device 46.As described above, when data are recorded, the calibration signals arestored in the optical recording medium 1 at predetermined density.

The pick up 43 further includes an objective lens (not shown) and a halfmirror (not shown) and condenses the laser beam set to the recordingpower or the reproducing power onto the recording layer 12 of theoptical recording medium 1. Concretely, the objective lens is focustracking controlled by the focus tracking servo 44, thereby condensingthe laser beam set to the recording power or the reproducing power ontothe recording layer 12 of the optical recording medium 1. The pick up 43is reciprocated by the feed servo 45 between the inner circumferentialportion and the outer circumferential portion of the optical recordingmedium 1 along the diameter direction thereof. The control device 46controls the operations of the spindle servo 41, the pick up 43, thefocus tracking servo 44 and the feed servo 45 and reads data recorded inthe recording layer 12 based on electrical signals output from the pickup 43 and the corresponding calibration signals.

As described above, according to this embodiment, since the calibrationsignals are stored in the optical recording medium 1 at predetermineddensity, it is possible to cancel the influence of the variation in thelight reflection coefficient of the recording layer 12 in the planethereof caused by forming the recording layer 12 of an organic dye usinga spin coating process. In particular, if the storage density of thecalibration signals is determined so as to increase with increasingproximity to the inner circumferential portion of the optical recordingmedium 1 and decrease with increasing proximity to the outercircumferential portion of the optical recording medium 1, it ispossible to simultaneously improve data reading accuracy data andincrease the area usable by the user.

The present invention is in no way limited to the aforementionedembodiments, but rather various modifications are possible within thescope of the invention as recited in the claims, and these are naturallyincluded within the scope of the invention.

For example, in the above described embodiment, although the virtualrecording cell train S00 consisting of ten successive unrecorded virtualrecording cells and the virtual recording cell train S77 consisting often successive virtual recording cells to which recording marks Mg areassigned are used as the virtual recording cell train 20 constitutingcalibration signals, the number of the virtual recording cellsconstituting the unrecorded virtual recording cell train S00 and thenumber of the virtual recording cells constituting the virtual recordingcell train S77 to which recording marks Mg are assigned are not limitedto ten and may be arbitrarily determined. However, as mentioned above,since the first virtual recording cells S00-start and S77-start and thelast virtual recording cells S00-end and S77-end are not suitable foruse as calibration signals, the number of the virtual recording cellsconstituting the virtual recording cell train S00 and the number of thevirtual recording cells constituting the virtual recording cell trainS77 have to be equal to or larger than three. On the other hand, in thecase where the number of the virtual recording cells constituting thevirtual recording cell train S00 and the number of the virtual recordingcells constituting the virtual recording cell train S77 are too large,the number of virtual recording cells usable for recording informationbecomes small. It is therefore preferable to determine the number of thevirtual recording cells constituting each of the virtual recording celltrain S00 and the virtual recording cell train S77 to be equal to orsmaller than 50. Further, it is not absolutely necessary for the numberof the virtual recording cells constituting the virtual recording celltrain S00 and the number of the virtual recording cells constituting thevirtual recording cell train S77 to be the same and they may bedifferent from each other.

Further, in the above described embodiment, although the virtualrecording cell train 20 constituting calibration signals is constitutedby the unrecorded virtual recording cell train S00 and the virtualrecording cell train S77 to which recording marks Mg are assigned, it isnot absolutely necessary to constitute the calibration signals by theunrecorded virtual recording cell train S00 and the virtual recordingcell train S77 to which recording marks Mg are assigned and thecalibration signals may be constituted by another combination of aplurality of virtual recording cell trains. For example, the calibrationsignals may be constituted by the unrecorded virtual recording celltrain S00 and a virtual recording cell train S44 to which recordingmarks Md are assigned or may be constituted by a virtual recording celltrain S11 to which recording marks Ma are assigned and a virtualrecording cell train S66 to which recording marks Mf are assigned.

Furthermore, in the above described embodiment, although the virtualrecording cell train S00 and the virtual recording cell train S77 arecontinuously formed, another virtual recording cell train may beinterposed therebetween.

Moreover, in the above described embodiment, although the opticalrecording medium 1 is constituted so that a laser beam is projectedthereonto from the side of the substrate 11 when data are to be recordedand when data are to be reproduced, the present invention can be appliedto an optical recording medium 1 constituted by sequentially forming areflective layer, a recording layer and a light transmittable protectivelayer in this order so that a laser beam is projected thereonto from theside of the light transmittable protective layer.

Further, in the above described embodiment, although three bits ofinformation are stored in one virtual recording cell S by formingrecording marks Ma to Mg in which the degrees of decomposition anddegradation are different from each other among seven stages in thevirtual recording cells S, the number of stages the recording marks isnot limited to seven and recording marks in which the degrees ofdecomposition and degradation are different from each other among anarbitrary number of stages may be formed insofar as one or more bits ofinformation can be recorded in one virtual recording cell S.

Furthermore, in the above described embodiment, although the explanationwas made as to the case where the present invention was applied to theCD-R type optical recording medium 1 (write-once type optical recordingmedium 1), as mentioned above the present invention can be applied to aCD-RW type optical recording medium (data rewritable type opticalrecording medium) in which a phase change material is used for arecording layer. In this case, since the recording layer is formed usinga sputtering process, it is possible according to the present inventionto cancel the influence of the variation in thickness of the recordinglayer in the plane thereof caused by the structure of a sputteringdevice, the influence of sputtering target consumption (erosion) as thesputtering process progresses, and the like.

As described above, since the calibration signals are stored in theoptical recording medium 1 according to the present invention atpredetermined density, it is possible to cancel the influence of thevariation in thickness of the recording layer in the plane thereofcaused by various factors.

1. An optical recording medium which comprises at least a recordinglayer and in which data can be recorded in a multilevel manner bycontrolling state of the recording layer among multiple stages, therecording layer storing a plurality of calibration signals along arecording track, each of the plurality of calibration signals includingat least a first virtual recording cell sequence in which plurality ofvirtual recording cells each having a highest reflection coefficient aresequentially formed and a second virtual recording cell sequence inwhich a plurality of virtual recording cells each having a lowestreflection coefficient are sequentially formed.
 2. The optical recordingmedium in accordance with claim 1, wherein the recording layer is formedof a dye.
 3. The optical recording medium in accordance with claim 2,wherein the recording layer is formed using a spin coating process. 4.The optical recording medium in accordance with claim 3, wherein densityof calibration signals stored at an inner circumferential portion ishigher than that of calibration signals stored at an outercircumferential portion.
 5. The optical recording medium in accordancewith claim 1, wherein the recording layer is formed of a phase changematerial.
 6. The optical recording medium in accordance with claim 5,wherein the recording layer is formed using a sputtering process.